This invention relates to the treatment of aluminum surfaces and more particularly to the surface treatment of an aluminum metallization system. Still more particularly, this invention relates to the treatment of aluminum metallization for silicon semiconductor devices.
The requirements for a material, or system of materials, to provide ohmic contact to and metallization stripes from semiconductor devices is extremely stringent from a mechanical, electrical and chemical view. Obviously, the first criteria must be that the material provide good electrical conductivity and ohmic contact to the semiconductor region. In standard planar types of semiconductor devices and integrated circuits, the material must not only make good ohmic contact to the silicon, but good mechanical contact to the silicon and to the silicon dioxide regions surrounding the contact area. Not only must the material bond well to the silicon oxide, but also, particularly in multilevel systems, to silica glasses or glasses of other types. These glasses should have good adherence to the material upon deposition thereon. Preferably, the material has a temperature coefficient of expansion closely matching the material in which it must be in contact. For manufacturing reasons, the material should be easy to deposit by standard evaporation or sputtering techniques and be easily patterned by etching or similar techniques. Since, in certain instances, it will be necessary to bond either gold or aluminum wires to the contact or metallization regions, these materials must be readily bondable to the material by standard thermocompression of ultrasonic bonding techniques. In use, the material should be mechanically strong, corrosion resistant and not subject to hillocking, electromigration, or similar thermal or electrical stress defects.
The only single metal previously found at all suitable for contacts and interconnections in silicon planar transistors and integrated circuits is aluminum, which material has been widely utilized for these purposes. The three most common problems attendant to the use of aluminum are:
1. Pitting of the aluminum in the silicon contact area. PA1 2. Electromigration. PA1 3. Its thermal expansion characteristics relative to that of silicon and/or silicon dioxide.
This latter characteristic results in hillocking of the metallization stripes which can crack the dielectric material, particularly in multilevel structures. To some degree the problems of electromigration and etch pitting of the silicon have been alleviated by the addition of small amounts of alloying impurities such as copper or silicon into the aluminum. A suggestion as a solution for hillocking has been annodizing of the aluminum surface. However, these methods have not been entirely successful, since hillocking is still found to occur, particularly when the devices are heated to or operated at elevated temperatures.